wip: Implement the constraint-based arrow notation proposal

* wip: Add ArrowEnv, ArrowStackTup, and ArrowEnvTup
* wip: Use arrow wired-in families in typechecker
* Rewrite arrow desugaring
* Fix some arrow typing issues
* Fix most of the remaining arrow tests
* Reorganize arrow-related modules
* Tidy up a few comments
* Accept new type error output for now

  I want to improve the type error involving ArrowStackTup to hide the
  family from the error message, but that can come later.
* Accept some more modified error output in tests

  These mostly arise from making Either a wired-in type.
* Update two test cases for new control operator typing
* Expand the wired-in arrow type families in type errors

  I am not sure that this is really the right place to do this
  expansion.  It seems like it should probably go in the
  tidying/printing code, but it would be a significant expansion of the
  responsibility of tidying, so I don’t know if that makes sense yet. I
  will probably change this later.
* Rename a few things and add some new Notes
* Add another Note; rename some things; various minor refactorings